Heat pipes are compact, passive heat transfer devices capable of handling high heat fluxes with a minimal temperature difference. Typical temperature differences are approximately 20° C. from heat source to heat sink. This versatile invention is employed in a wide range of applications, from spacecraft temperature control, waste heat recovery and microelectronics cooling to other applications where rapid heat dissipation is required. A heat pipe's heat transport capacity is highly dependent on the properties of the porous wicking structure, the material that provides the capillary pumping action necessary for working fluid transport. An optimum wick will provide high capillary pumping pressure with good permeability for minimal flow resistance. Both properties are dependent on the characteristics of the wick structure. For example, capillary pressure increases as pore size decreases, yet permeability is adversely affected at the same time.
Presently, homogeneous wick structures (ie: wicks with one nominal pore size) have the disadvantage of compromising between these characteristics to achieve suitable performance. However, hybrid wicks offer a possible solution: a structure with bi-modal pore sizes can offer both high capillary pumping pressure and excellent permeability, thus satisfying both criteria simultaneously. Additionally, depending on its location within a heat pipe, there are different performance requirements imposed on the wick material. For example, since the capillary pumping pressure is mostly affected by the pore size of the wick in the evaporator section, it is desirable to have a finer pore size at the evaporator and a coarse pore size at the condenser. A smooth transition between these two sections is also necessary to minimize end-to-end pressure drops. The focus of this invention is the development of new hybrid wicking materials with optimally combined fine and coarse porous structures for heat pipe applications. Other fields, where performance is primarily dependent on the optimization of porous materials, in the sense of enhanced heat transfer or otherwise, will also benefit from these hybrid porous structures.
A heat pipe is a heat transfer device with high thermal conductivity. It consists of a sealed container, a porous wicking material and a working fluid. Air is evacuated from the container and an adequate amount of working fluid is added to fully saturate the wick. The container is then sealed and the heat pipe is connected to a heat source and heat sink. Examples of heat pipe devices are described in U.S. Pat. Nos. 3,152,774 and 3,229,759.
Heat pipe applications reflect their remarkable versatility. Thermal management of microelectronics and spacecraft temperature control are two main areas where the application of heat pipes is extensively reported. Other notable industrial uses include the preservation of permafrost in pipeline applications and thermosyphons used in nuclear power generators.
Porous, sintered (U.S. Pat. No. 4,885,129) or plated (U.S. Pat. No. 4,311,733) wicks are commonly used in electronics cooling applications. Other wick structures such as wrapped metal screens or felt and axial grooves are also used, however their reduced capillary pumping ability hinders operation in unfavourable orientations (ie: against gravity operation). Additionally, heat pipes constructed from wrapped screen or felt material are susceptible to developing hot spots impeding or blocking liquid movement. This phenomenon occurs because poor thermal contact between the wick and the inside wall of the container are potential bubble nucleation sites which impede heat transfer. Heat pipes for spaceflight applications typically use axial grooves as wicks with ammonia as the working fluid. However, sintered metal copper or nickel wicks are also used in the loop heat pipe designs. Other types of heat pipes were also reported to contain U-shaped heat pipes with sintered copper wicks and acetone as the working fluid. To further enhance the performance of heat pipes containing grooves, hybrid grooved structure has been proposed and fabricated with both axial and circumferential (radial) channels as described in U.S. Pat. No. 5,335,720.
In order to increase the effective heat transfer rate, several modifications have been proposed to improve homogeneous wick structures. In the following references, U.S. Pat. Nos. 6,880,626, 4,274,479 and 4,929,414, a vapour chamber with grooves or arteries in combination with sintered powders claimed to improve both capillary pumping performance and film boiling resistance. However, complex fabrication techniques were involved and required the formation of grooves or arteries by machining, powder preform or sacrificial polymer lines prior to sintering. Additionally, these fabrication methods are not capable of axially or radially varying the porous structure through the wick.
In U.S. Pat. Nos. 5,101,560 and 4,964,457, metal powder was sintered onto a wire screen under a magnetic field to make an anisotropic wick structure for unidirectional heat pipes. In this disclosure, the purpose of the wire screen was as a support/carrier for the metal powder. The wire screen itself was not used to enhance heat transfer performance and metal powder coverage onto the screen was not intentionally controlled to provide optimized heat transfer performance. Some success has already been demonstrated using this idea. A composite, layered stainless steel wick comprised of highly permeable metal screens and a fine pore sintered metal powder layer was shown to perform better than either a screen or sintered wick alone, as disclosed in Canti et al., Thermal Hydraulic Characterization of Stainless Steel Wicks for Heat Pipe Applications, Rev. Gen. Therm., V37, pp 5-16, 1998. However, this structure is not ideal since the presence of layer interfaces could introduce vapour traps which may impede liquid flow through the wick.
A composite heat pipe is detailed in U.S. Pat. No. 4,565,243. The wick in the evaporator region is constructed from sintered metal powder, while the wick in another region of the heat pipe consists of a screen to permit bending of the pipe without destruction of the sintered powder. In order to remove thermal energy during power surges, a thicker wick is incorporated into the sections adjacent to the evaporator. This feature provides an extra fluid reservoir, as described in U.S. Pat. No. 4,674,565. As seen from the above survey, hybrid wick structures have been proposed in a number of similar applications. However, they are significantly different from what is disclosed in this invention.
Other related composite structures for non-heat pipe applications are also reviewed. In U.S. Pat. Nos. 6,719,947, ceramic or metal foam was used as structural support for sintered powder material. Since all pores in the foam were covered with sintered powder, the resulting material could only assume a single nominal pore size. Furthermore, the material was intended for filtration applications only.
Similar to the previously described patent, an organic composite porous material has also been proposed (U.S. Pat. Nos. 4,732,887, 5,814,372, 6,306,488, 6,569,495, and 6,815,050). This patent consists of a porous cellulose material with its large pores covered with a secondary porous polymeric material. The purpose of the secondary material is to provide mechanical strength and dimensional stability while at the same time permitting fluid entry into the primary cellulose material for filtration or extraction applications.
U.S. Pat. No. 6,648,063 discloses a perforated metal plate which is added onto a sintered stainless steel felt, its purpose being to provide structural support.
Thus, it is evident that widespread heat pipe applications along with ample opportunities for advancement in wick design create the need for novel porous wick materials.